The present invention relates generally to magnetic resonance (MR) imaging systems, and more particularly, to an apparatus and system for performing high field MR scanning and imaging of a body. In magnetic resonance imaging, a patient is typically positioned within a strong, temporally constant or static magnetic field, referred to as the B0 field. A time series of magnetic field gradient pulses, for encoding spatial location, are applied across a region of interest within the magnetic field. Concurrently, radio frequency (RF) pulses are applied to induce and manipulate the magnetic resonance of magnetic dipoles in the region of interest. An assembly of RF transmitting and receiving coils is positioned over portions of the patient to excite and receive the radio frequency magnetic resonance signals within the region of interest. The RF magnetic field, generated by the RF pulses, is referred to as the B1 field.
The MR imaging systems that use a low B0 field (0.5 T or less) have a low B1 field frequency or Larmor frequency. As such, the electric field associated with the B1 field is negligible and the interaction between the B1 field and the patient can be neglected.
Due to the signal-to-noise ratio (SNR) limitations of a low B0 field MR imaging system, MR imaging systems using a high B0 field (3.0 T or greater) have been developed. However, as the strength of the B0 field increases, the frequency of the B1 field increases linearly and at such a high frequency (100 MHz or greater), the interaction between the B1 field and the patient can no longer be neglected. This interaction is caused by the effective wavelength of the B1 field, at the higher frequencies, being comparable to or even smaller than the dimension of the human body.
Such a strong interaction substantially degrades the homogeneity of the B1 field and thus negatively affects the image quality of the system and can cause an increase in image shading. Also, since the varying electric field strength associated with the B1 field increases with the inhomogeneity of the B1 field, the specific energy absorption rate of the patient increases or in other words patient heating increases. Increased patient heating can result in abrasions or burns to patient tissue. In addition, the high static fields reduce coil efficiency and increase power requirements.
Thus, there exists a need for an improved apparatus and system for performing high field MR scanning and imaging of a body that minimizes interaction between the electromagnetic (E) and B fields and the patient, maintains or improves image quality, maximizes slice coverage, and allows for increased imaging speeds.